(1) Field of the Invention
The present invention relates in general to improvements in a therapeutic system for producing automatic intermittent compression to minimize or prevent deep vein thrombosis (DVT). More specifically, it relates to a system including a source of intermittent pressure and one or more pressurizable chambers attached to a human leg for providing treatment by applying intermittent compression to the leg by means of the chambers to accelerate the flow of venous blood and thereby minimize the risk of or prevent DVT. In a preferred embodiment, two or more chambers are used and the intermittent compression may be graduated along the body portion to which it is applied, and may also be applied sequentially to the two or more chambers.
(2) Background of the Invention
Therapeutic intermittent pneumatic compression of the leg for the prevention of DVT after surgery has been used for more than twenty years, and a variety of devices, many patented, have been developed for its application. Even with these devices, the incidence of DVT remains relatively high and the use of existing equipment is somewhat limited because of its high cost and patient discomfort. A clear and well recognized need exists for a system that is more effective, less costly, and more comfortable for the patient.
Intermittent pneumatic compression is the technique of cyclically compressing the limb with air pressure so as to enhance circulation of blood. It has been shown effective in reducing the risk of thrombosis after surgery and for treatment of vascular deficiencies. The pressure is applied from a source of compressed air by a control mechanism that intermittently inflates a cuff enveloping the arm or leg. The period of compression is typically short, ten seconds or so, and the interval between pulses about a minutes, studies having shown this to be the time required for the veins to refill after being emptied by the short pulse of compression.
Studies have also shown that the optimal amount of compression is in the range of 35 to 45 mm hg, and that the velocity of the venous flow during the period of compression is proportional to the rate at which the pressure rises. For example, a pulse that rises to 35 mm hg in six seconds accelerates venous velocity by several times that of a pulse that requires 30 seconds. Because it is this acceleration of venous flow that is believed to reduce the risk of pooling and clotting of blood in the deep veins, the rate of pressure rise is a critical variable of effectiveness in reducing the risk of or preventing DVT. Cost and comfort are also variables, because the more expensive and less comfortable devices are less likely to be used.
It has been shown that intermittent pneumatic compression (IPC) is more effective in preventing DVT when there is a higher velocity of venous flow during the period of compression. Graduated intermittent pneumatic compression of fluid chambers applied to an injured body portion is well known and its efficacy is broadly accepted by the medical profession. With graduated IPC, pressures of different magnitude are applied to different regions of the body portion being treated. Most typically higher pressure are applied to the distal regions and lower pressures are applied to the proximal regions. IPC may also be applied sequentially, with the distal region of the treated limb being pressurized slightly before the proximal region. It also has been reported that a combination of sequential and graduated compression may be the most effective in providing accelerated venous flow.
Roberts, et al., "Hemodynamics of the Lower Limb in Man," Brit. J. Surg., Vol. 59, No. 3, pp. 223-226, March 1972, reports that intermittent pressure applied with an inflatable plastic splint causes an increase in venous peak flow directly proportional to the rate of pressure application, being maximal at about 10 mmHg per second, with the maximum being reached when the pressure is applied at 1 minute intervals.
Nicolaides, et al., "Intermittent sequential pneumatic compression of the legs in the prevention of venous stasis and postoperative deep venous thrombosis," Surgery, Vol. 87, No. 1, pp. 69-76, January, 1980, discloses tests with a multi-chamber sequential pressure device. Optimal pressures were found to be 35 mmHg for the ankle, 30 mmHg for the calf, and 20 mmHg for the thigh, which produced a 140% increase in blood velocity, higher pressures did not cause any increase in blood velocity.
Salvian, et al., "Effects of intermittent pneumatic calf compression in normal and postphlebitic legs," J. Cardiovasc. Surgery, 29, 1988, pp. 37-41, evaluated two sequential compression devices and one single chamber device. Correct calf application was found to be critical in achieving an increase in blood velocity.
Most of the systems known for producing IPC are large, expensive, complex and uncomfortable for the patient. For example, in U.S. Pat. No. 4,013,069, a system is disclosed that treats deep vein thrombosis in the thigh, the calf, and the ankle. This prior art system is accomplished with an elaborate system of multiple lines and mechanical valves and a multiplicity of tubes leading from a complex control system to the air cells about the leg. The system also is complicated and apparently requires four timers, three pneumatic shift valves, and a separate tube from the controllers to each of the six pressure zones. It also requires means for intermittently initiating periodic deflation cycles at the end of inflation cycles. As will be seen, this is opposite to the system of the present invention, in which the pump operates continuously and the air cells exhaust automatically, at a rate great enough to permit rapid pressure drop.
There is a need for a device that provides graduated IPC and that is small, lightweight, economical to construct and operate, comfortable for the patient and efficient to use.
(3) Objects of the Invention
The principal objective of the present invention is to provide a system for intermittent pneumatic compression that provides faster inflation, less complexity, lower cost, and greater patient comfort. These seemingly contradictory goals are achieved by employing unique strategies for the way in which the compressed air is accumulated and released to inflate the cuff; for the way compression is applied to the leg, and for the design of the cuff to facilitate inflation.
It is an object of the invention to provide a system for applying therapeutic intermittent pneumatic pressure to a limb, wherein said pressure is applied with rapid acceleration to effect therapeutic venous flow acceleration.
It is yet another object of the invention to provide a system for applying therapeutic intermittent pneumatic pressure to a limb, wherein said pressure is applied with more rapid acceleration than in prior art designs, to thereby effect therapeutic venous flow acceleration, and wherein said system is of relatively simple construction and relatively low cost.
It is yet another object of the invention to provide such a system for applying therapeutic intermittent pneumatic pressure to a limb, wherein said system affords greater patient comfort than prior art systems.
It is still another object of the invention to provide a system which provides graduated, sequential therapeutic intermittent pneumatic pressure to a limb.
Yet another object of the invention is to provide the entire system in a conveniently arranged carrying device which accommodates easy mobility and set up of the system.
Other objects, advantages, and novel features of the instant invention will be readily apparent to those skilled in the art from the following description and drawings.